I have a few additional comments to add to kscharf's. The reason these small sealed subs typically have so much power is to bass boost their otherwise high F3 down to a level most aficionados expect to see. Another of my hobby interests is speaker building, and I've yet to find a 10" driver that provides an unassisted F3 under 40 Hz in a sealed box. Typically the unassisted F3 will be in the mid-40's at best. So, this type sub will need 12 dB of boost to achieve another octave lower performance. That's 16 times more power that does nothing to make the overall output louder. Dissipating all that extra power in the voice coil generates a lot of extra heat which leads to thermally-induced distortion. And then that small driver will need a longer excursion which adds to non-linearity distortion. The small box pleases a lot of customers, but it comes at a price. Of course nearly everything in life is one compromise or another, and you have to make the ones that suit you best.

A Linkwitz transform filter will compensate one Q design to another. This technique works for both ported and sealed designs to allow using a smaller than optimum box while still yielding a Q of 0.7 or lower. Rythmik Audio provides switch-selectable Q on their subs (0.5, 0.7 & 0.9) using Linkwitz transform.

The most common HT sub crossover implementation (concurring with the THX spec) rolls off the sub hi end at -24dB per octave while the main speakers' low-ends are rolled off at -12 dB per octave. This seems to expect main speakers with an F3 at the sub crossover frequency with a -12 dB per octave rolloff to provide equal effective high and low pass responses of 24 dB per octave. That requires sealed main speakers. I have not evaluated whether this makes an appreciable difference in SQ, but it did persuade me to switch to sealed design speakers when I upgraded recently. Seemed like one less compromise might be a good thing.

Incidentally, my upgrade did include dual sealed subs to eliminate another compromise. After some research, I decided to go with Rythmik Audio F15 servo subs. I'm sure there are better subs out there, but I'm thrilled with the F15 musical performance, and they're no slouch on HT either. I'll be keeping these awhile.

Bill